The present invention relates to a sensor having a reference electrode, more specifically, a reference electrode with an internal reference junction. Reference electrodes are commonly used in connection with ion-selective electrodes to determine the concentration of ions in solution. For example, a reference electrode is often used with an electrochemical ion-measuring electrode, such as a glass pH electrode, to measure the concentration of hydrogen ions in a process solution. In particular, the present invention relates to sensors for measuring the ion concentration of a process solution, e.g., fluids, slurries, and the like.
The basis of the electrometric measurement of pH is the development of a potential gradient across a membrane of specific composition, when interposed between solutions having different concentrations of hydrogen ion. The potential developed across the membrane is quantitatively related to the concentration gradient of hydrogen ion and can be applied to a known measuring circuit to measure the pH of the sample. Because the potential developed across the glass is to be measured, electrolytic contacts must be made to the solutions on either side of the membrane. The potentials generated by these contacts are controlled using, for example, Ag/AgCl reference electrodes with controlled concentrations of potassium chloride (KCl) solution.
The conventional, external reference electrode has two components that contribute to the total potential measured across the cell: a thermodynamic potential and a liquid junction potential. The thermodynamic potential is derived from the electrochemical half-cell, whereas the liquid junction potential is derived from the difference in the ionic composition of the internal salt bridge electrolyte and the process solution being measured. For example, where the reference electrode half-cell reaction is:
Ag+Clxe2x88x92⇄AgCl+exe2x88x92
the potential generated may be fixed by: (1) controlling the concentration of chloride ion, i.e. Clxe2x88x92, at a constant value; and (2) preventing interfering ions in the process solution from approaching the reference half-cell. In prior reference electrodes, these conditions are typically achieved by filling the reference electrode with potassium chloride (KCl), often within an internal chamber, which is connected to a salt bridge using an internal ceramic barrier. In such electrodes, electrolytic contact between the salt bridge and the process solution is made via an external ceramic barrier, and the salt bridge is stationary, i.e. non-flowing. In this configuration, both the liquid junction and the half-cell potential may be compromised during ingress of the process solution into the internal salt-bridge and reference half-cell solutions. Thus, accurate measurements require that cell voltage varies only with the concentration of the ion of interest, and that the reference electrode potential remain constant, i.e. unaffected by the composition of the process solution. In fact, it is known that the reference electrode is often the cause of poor results obtained from measurements with ion-selective electrodes. See Brezinski, D. P., Analytica Chimica Acta, 134 (1982) 247-62, the contents of which are hereby incorporated by reference.
In addition, the development of process sensors has tended toward probes with smaller diameters. This trend has made the construction of highly accurate and stable sensors more difficult. For example, in certain sensor designs, positioning the reference electrode further away from the process solution has resulted in decreased accuracy, due to decreased thermal accuracy. Thus, it would be desirable to have a sensor with increased stability and accuracy of measurement which decreases or eliminates the ingress of process solution. There is also a need for improved sensors having smaller diameters while also minimizing the process-wetted portion of the sensor.
In view of these considerations, it is an object of this invention to provide a reference electrode that minimizes or prevents back-flow of contaminants or materials from the process solution through the external junction. It is also an object of this invention to provide a durable, economical and versatile reference electrode that is easy to fabricate, use, install, calibrate and maintain. These and other objects are satisfied by the invention described herein.
The present invention provides a sensor with a reference electrode and a flowing electrolyte. The invention provides sensors that operate with relatively high accuracy and stability by minimizing or eliminating ingress of contaminants from a process solution through the external junction of the sensor. In one aspect, the invention includes a sensor having a pressurized reservoir which provides flow of an electrolyte. In another aspect, the invention provides a sensor having a non-metallic solution ground. In yet another aspect, the invention includes a resistance temperature device bonded to a non-metallic solution ground.
In one embodiment, the invention provides a sensor having a reference electrode, a flowing electrolyte in electrolytic contact with the reference electrode, a pressurized reservoir for providing flow of the electrolyte, a reference junction and an external junction in electrolytic contact with the reference electrode and wherein the electrolyte flows between the junctions.
In another embodiment, the invention provides a sensor having a reference electrode, a flowing electrolyte in electrolytic contact with the reference electrode, a pressurized reservoir for providing flow of the electrolyte, and a non-metallic ground disposed at a sensing surface.
In yet another embodiment, the invention provides a sensor having a reference electrode, a flowing electrolyte in electrolytic contact with the reference electrode, a pressurized reservoir for providing flow of the electrolyte, a non-metallic ground disposed at a sensing surface, and a resistance temperature device bonded to the non-metallic ground.
Sensors of the invention may be used to measure various parameters of a fluid, e.g., ion concentration. In one preferred embodiment, the sensor is a pH sensor, i.e. a sensor to measure hydrogen ion concentration, having a reference electrode, a flowing electrolyte in electrolytic contact with the reference electrode, a pressurized reservoir for providing flow of the electrolyte, a reference junction, and an external junction in electrolytic contact with the reference electrode. The electrolyte flows from the pressurized reservoir to the external junction. In another preferred embodiment, the pH electrode includes a non-metallic ground disposed at a sensing surface. In yet another preferred embodiment, the pH sensor includes a resistance temperature device bonded to the non-metallic ground. In a particularly preferred embodiment, the non-metallic ground extends beyond the end of the lower housing and, even more preferably, the non-metallic ground is substantially conical in shape.
In still another embodiment, the invention provides a method of manufacturing a sensor having a resistance temperature device and a non-metallic ground, the method including the steps of melting the non-metallic ground in contact with the resistance temperature device and allowing the non-metallic ground to solidify in contact with the resistance temperature device, thus ensuring optimal thermal contact. In yet another embodiment, the invention includes a method of manufacturing a sensor having a resistance temperature device and a non-metallic ground that is substantially conical in shape.